Another nav's point of view......
I was deselected very early on on thanks to a crippling lack of coordination and inability to learn at the rate required. You only need to watch me on a sports field or court to realise! So nature seemed to play a big part in that. However, I then did well through nav training, nature had given me the right tools for that.
Interestingly, I did my second tour at 6FTS where shortly after I had got my B1 I had a conversation with OCANS who told us that HQ RAFSC had decided that in future all baby navs would come from the pilot training system failures to save money on training (ie already done most of the groundschool and some visual nav and would be therefore ahead of the game). By this time most navs were bound for a FJ or Kinloss. After a bit of dragging through records we produced some stats that this was counter intuitive, if a guy was chopped after spin aeros (ie he could fly but not operate the aircraft) he would get a bit further in the nav training but would ultimately fail as he couldn't crack the high tempo 'mind games'. A guy off the street who had passed the Nav side of OASC had a much better chance as did the uncoordinated duffers like me.
So ultimately, with the Tornado era you had to have two lots of nature to crack it as a FJ pilot, plus a good deal of hard work unless you were one of the very lucky ones.